The use of a melt-out metal core of complex shape to provide a detailed internal configuration to a subsequently moulded part of plastic material is an area of developing technology, especially in the automobile industry. Such cores are made of a low melting point alloy and are removed from the moulded component by melting.
The prime requirement for these metallic cores is that they should provide accurate dimensional forms, as well as predetermined surface finishes. The metals from which such cores can be made have solidus temperatures in the range of 35.degree. to 300.degree. C.
There are a number of established methods of casting such alloys, ranging from simply pouring the liquid metal into a suitable metallic or non-metallic mould, either by hand or mechanically, through a range of various pressure devices to introduce the metal into the mould cavity, examples of which are centrifugal rubber mould casting, low pressure gravity diecasting, high pressure diecasting, and the Durville casting method.
For the present application it has been found by experience that none of the available techniques provides castings with specific required characteristics of dimensional tolerance, surface finish and lack of internal porosity or cavitation. While diecasting as normally practised produces a good surface finish, there is a tendency to provide porosity in the castings which is unacceptable in the above-mentioned cores.
The object of the invention is to enable low melting point alloys with solidus temperatures in the range 35.degree.-300.degree. C. to be accurately and reproduceably cast.